This summary of the tale is taken from here - Metamorphosis project
The Story of Túan mac Cairill
This text, which dates from around the ninth-century, is a story in which Túan, who lives in a hermitage, tells his story to a monk named Finnia, revealing that he arrived in Ireland along with the very first settlers, who subsequently died of a plague, with Túan being the sole survivor. Túan then spent time living alone in the wilderness and grew old:
‘Then I was fleeing from refuge to refuge and from cliff to cliff, protecting myself from wolves. Ireland was empty for thirty-two years. Age came upon me at last, and I could no longer travel. I was in cliffs and in wildernesses, and I had caves of my own.’
Then the next wave of settlers landed:
‘The son of Agnoman landed, my father’s brother. I used to see them from the cliffs, and hid from them: I was shaggy, clawed, wrinkled, naked, wretched, sorrowful. I was asleep one night. I saw that I went into the shape of a wild stag. I was there thereafter: I was young, and in good spirits, and the lord of a herd, and I made a circuit of Ireland with a great herd of stags around me.’
It was during this time, whilst Túan was in the shape of a stag that Nemed and his followers settled Ireland. They too didn’t survive and all died.
Then again, Túan reveals:
“Age came upon me at last, and I fled from men and wolves. I stood one night at the entrance of a cave. I remembered, and I knew how to go from one shape into another. I went into the shape of a wild boar. I found that swift, then, and I was in good spirits, and I was lord of the boar-herds of Ireland, and I used to make a circuit of Ireland. And I had furthermore a dwelling in this region of the Ulaid, which I visited in the time of my old age and wretchedness. For it is in one place that I used to change all these shapes.’
It was during his time in the shape of a boar that the next waves of settlers arrived into Ireland, The Fir Domnann and the Fir Bolg. But then eventually:
‘Age came-upon me, and my mind was sorrowful, and I could not keep company with the boars and the herds, but was alone in caves and cliffs. I went ever to my dwelling. I remembered every shape in which I had been. I fasted my three days’ fast. I had no strength. I went into the shape of a hawk. I was content with that. My spirit was very mighty. I was happy, eager. I flew across Ireland. I learned all things.’
During Túan’s time as a hawk the Túatha Dé Danann arrived in Ireland, followed by the Gaels. Then Túan grew old as a hawk:
‘Once I journeyed in the hawk’s shape, in which I was, to the hollow of a tree, above a stream. My mind was sorrowful. I could not fly, and I feared other birds. I fasted an ennead [a time interval of nine days, months, or years] then, and went into the shape of a fresh-water salmon. God puts me into the river. That was wondrous for me then, and I was vigorous and happy, and I was a master of swimming. I escaped from every peril: from the hands of fishermen and from the claws of hawks and from the spears of fishers, so that the wounds of them are in me.’
Túan was eventually caught by a fisherman and was eaten by the wife of Cairell, king of Ulster. Túan ended up in the queen’s womb and nine months later was born as Túan son of Cairell who remembered everything that had happened to him since he first arrived in Ireland through his successive changes into a stag, boar, hawk and salmon. In this wise, he was able to reveal to the learned Finnia the entire history of the invasions of Ireland.
John Carey, ‘Scél Tuáin Meic Chairill’, in Ériu Vol. 35 (1984), pp. 93–111.